This invention relates to the precision testing of electrical cable, and particularly to measuring the length of a cable that is used for electronic communications.
In the installation and maintenance of communications cables there are many parameters that need to be known to the craftsman in order for him to be able to accomplish his work most rapidly and correctly. Electrical cables come in many different sizes and shapes. A cable can be twisted pair, coaxial, ribbon (or so-called xe2x80x9csatinxe2x80x9d), Romax, or any similar type. Cables are usually made of consistent geometry over their entire length. The tests for possible short circuits, open circuits, and incorrect conductor pairing have long been standard. Conductor paths are also traced to determine whether two cable ends belong to the same cable.
In recent years instrumentation has been provided that allows a craftsman to make a number of different tests with a single test instrument. The instrument is connected and turned on, its internal computer then goes into operation, and its visual and/or other outputs can then rather instantaneously provide results of a sequence of different tests.
One measurement that is often needed is the length of a cable. Measuring length is often difficult, especially when the cable is routed through building walls and floors, underground, or intertwined with other cables, and is not visible throughout its route. Several different methods have been known. The present invention provides a method that can be used at only one end of a cable for precisely measuring its length.
U.S. Pat. No. 5,339,022 issued Aug. 16, 1994.
According to the present invention the capacitance per unit length of the cable, if not previously known, will first be determined. The present invention applies to a cable that has a uniform physical characteristic throughout its length, and hence also has presumably uniform capacitance per unit length.
According to the novel method of the present invention a reference capacitor of known value and one end of the cable under test are alternately inserted into a stable oscillator circuit. The two resulting oscillator frequencies are measured and compared, and their ratio is then used to calculate the cable length. The two measurements are made in rapid succession, preferably within a fraction of a second, so most variables that influence the oscillator frequency will cancel out in determining the final length measurement.
Further according to the invention, the oscillator frequency is determined by counting pulses during a predetermined time period, and the output signal is converted into sharpened pulses for better accuracy in the counting process.
As a further feature of the invention, the length of the predetermined time period during which oscillator output is observed may be adjusted or varied in accordance with the length of the cable, to improve the accuracy of measurement.
The object of the present invention, therefore, is to rather precisely measure the length of a cable from only one of its ends, in a fast and accurate manner.